This invention relates to a process for preparing a YBaCuO system superconducting precursor powder and particularly, to a process for preparing a fine and homogeneous YBaCuO system superconducting precursor powder by an oxalate route coprecipitation method using a dibasic oxalate ester as a precipitate forming agent.
The coprecipitation process for preparing a YBaCuO system superconducting precursor powder via an oxalate route is known in the art. U.S. Pat. No. 4,895,832 discloses a process in which nitrates of Y, Ba and Cu are dissolved in water and oxalates of Y, Ba and Cu are coprecipitated by adding oxalic acid to the aqueous solution of the nitrates. In an article, "Solid State communications, vol. 64, No. 6, pp.881-883, 1987 ", Wang et al disclose a process for preparing a superconducting precursor powder also via the oxalate route, wherein oxalates of Y, Cu and Ba are coprecipitated by adding an aqueous solution of oxalic acid to a mixture containing an aqueous solution of copper acetate plus yttrium nitrate and a solution of barium hydroxide plus acetic acid. Oxalic acid permits oxalates of Y, Cu and Ba to precipitate rapidly. It is, however, ineffective to achieve a fine and uniform superconducting powder.
Japanese Patent Application No. 63-285116 discloses an oxalate route coprecipitation process in which oxalates of Y, Ba and Cu are precipitated by adding an organic dibasic ester such as dimethyl or diethyl oxalate to an aqueous solution containing ions of Y, Ba and Cu. It is described in this patent that the process produces fine and uniform coprecipitates because ethyl oxalate gradually decomposes to deliver oxalate ions used to form oxalate precipitates. However, this process is found to be not so efficient as excepted because dibasic esters such as ethyl oxalate are not readily soluble in water. It is difficult to achieve a homogeneous phase when mixing ethyl oxalate and the aqueous solution so that a long period is required for the decomposition of ethyl oxalate to an efficient state. The decomposition rate of ethyl oxalate is so undesirably slow that oxalate particles formed priorly coagulate to large particles, resulting in disuniform precipitate particles. This process does not use a base to adjust the pH value of the solution and thus desired components in the solution are liable to be lost thereby adversely affecting the desired stochiometry of the final product.